Mechanical structural part and method for manufacturing same

Abstract

It is provided a mechanical structural part and a method of manufacturing the same. The mechanical structural part comprises a chemical composition containing C: 0.45% to 0.51%, Si: 0.15% to 0.35%, Mn: 0.60% to 0.90%, P: 0.030% or less, S: 0.025% or less, Al: 0.040% to 0.059%, Cr: 0.10% to 0.50%, and N: 0.0060% to 0.0100%, with the balance being Fe and inevitable impurities, and a hardened layer by induction hardening and tempering treatment, wherein an area ratio of crystal grains each having a prior austenite grain size of 80 μm or less in the hardened layer is 80% or more, and a number ratio of grains each having a grain size twice or more than a mode of grain size in the hardened layer is 5% or less.

1 . A mechanical structural part comprising:
 a chemical composition containing, in mass %,   C: 0.45% to 0.51%,   Si: 0.15% to 0.35%,   Mn: 0.60% to 0.90%,   P: 0.030% or less,   S: 0.025% or less,   Al: 0.040% to 0.059%,   Cr: 0.10% to 0.50%, and   N: 0.0060% to 0.0100%,   
       with the balance being Fe and inevitable impurities, and
 a hardened layer by induction hardening and tempering treatment, wherein an area ratio of crystal grains each having a prior austenite grain size of 80 μm or less in the hardened layer is 80% or more, and a number ratio of grains each having a grain size twice or more than a mode of grain size in the hardened layer is 5% or less. 
 
     
     
         2 . The mechanical structural part according to  claim 1 , wherein the part has a shaft-shaped portion. 
     
     
         3 . A method for manufacturing a mechanical structural part, the method comprising subjecting a steel material having a chemical composition containing, in mass %,
 C: 0.45% to 0.51%,   Si: 0.15% to 0.35%,   Mn: 0.60% to 0.90%,   P: 0.030% or less,   S: 0.025% or less,   Al: 0.040% to 0.059%,   Cr: 0.10% to 0.50%, and   N: 0.0060% to 0.0100%,   
       with the balance being Fe and inevitable impurities to hot rolling at a rolling speed VSL satisfying the following formula (1) to form a steel bar or wire rod, and forging the steel bar or wire rod to be subjected to induction hardening at 900° C. to 1150° C. and then tempering: 
       
         
           
             
               
                 
                   
                     
                       VSL 
                       ≤ 
                       
                         100 
                         / 
                         DL 
                         ⁢ 
                            
                         
                           ( 
                           
                             m 
                             / 
                             s 
                           
                           ) 
                         
                       
                     
                     , 
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
         where VSL is a rolling speed (m/s) just before passing through a final stage of rolling, and DL is a diameter (mm) of a rolled material after the rolling is completed.